The present invention relates generally to the manufacture and testing of integrated circuit (IC) devices and, more particularly, to a test structure for determining optimal seed and liner layer thicknesses for dual damascene processing.
The reliability of copper (Cu) interconnects in IC devices is typically limited by failure mechanisms such as electromigration and stress migration, for example. In electromigration, Cu atoms migrate in the direction of the electron flow, eventually causing a void in the Cu lines. In stress migration, Cu atoms diffuse to relieve the thermal stress caused by the mismatch in the coefficient of thermal expansion (CTE) between the Cu and the surrounding dielectric material. In this case, void formation is also possible if sufficient vacancies are available. For dual damascene processing, in which both vias and lines are formed in the same processing step, liner materials are used for improved yield and reliability. Thus, where voids happen to be present in the Cu vias and/or lines, an open circuit may be prevented by maintaining a current path through the conductive liners. The same failure mechanisms are known to occur in aluminum (Al), gold (Au) and silver (Ag) interconnects.
Typically, such liner materials include, for example, tantalum (Ta), tantalum nitride (TaN), titanium (Ti), titanium nitride (TiN) and tungsten (W). Since these materials are significantly more resistive than Cu, the liner thickness is chosen such that the line resistance is not severely degraded. For electroplated Cu, a seed layer must be deposited into the via and line openings following liner deposition. The Cu seed is typically deposited by, for example, physical vapor deposition (PVD) prior to the plating process. The seed must be thick enough to ensure that a continuous layer forms over the entire wafer. Problems arise if the liner is made too thick, as the metal fill may be adversely affected if the openings are pinched off at the top of the vias and lines. Similar problems may occur if the Cu seed layer is too thick. Therefore, it is beneficial to have an optimized thickness of the Cu seed and liner such that both process and reliability improvements are possible.